Conventionally there is a sound reproduction device for transmitting sound information only to an object person by using a speaker which gives directionality to sound information including information of sound such as voice. For example, many information transmitting devices each provided for a vehicle to transmit information such as alarm sound only to a driver are proposed (refer to, for example, patent literature 1).
FIG. 13 is a layout of speakers of a conventional information transmitting device. Speakers 107 are installed at a base of door mirrors 103 below front pillars 101 on right and left sides of a vehicle body. Speakers 107 generate an ultrasonic wave toward the head position of driver 105.
The vehicle measures distance to an obstacle and transmits alarm sound to driver 105 in accordance with the distance. Right and left speakers 107 output ultrasonic waves of different frequencies. Right and left speakers 107 are disposed so that directivity axes indicative of travel directions of the ultrasonic waves emitted toward driver 105 cross in the head position of driver 105. The frequencies of the ultrasonic waves output from right and left speakers 107 are set so that frequency of a beat which is caused by mutual interference of the two frequencies belongs to an audible range. Therefore, the ultrasonic waves of the two difference frequencies become audible sound around the head of driver 105, and information can be transmitted as alarm sound to driver 105. On the other hand, around an occupant in another seat, the directivity axes of right and left speakers 107 do not cross and no beat is generated by the beat caused by the two ultrasonic waves, so that the alarm sound is not perceived. Since the directivity can be given to the alarm sound in such a manner, the alarm sound can be transmitted only to driver 105.
In the conventional information transmitting device as illustrated in FIG. 13, to transmit information as the alarm sound only to driver 105 among vehicle occupants, speakers 107 which emit an ultrasonic wave having high directivity are used. Consequently, even in the case of giving alarm to driver 105, the alarm sound is perceived only by driver 105, and the other occupants do not hear the alarm sound and, therefore, are not bothered.